As is known well, a motion in a plane has three degrees of freedom. Generally, the motion is realized by the use of three joints. For example, positioning of a work on a machining center is practiced such that a slide table in a Y-direction is mounted on a slide table in an X-direction, and a rotary table in a .theta.-direction is mounted on an assembly including the slide tables in the X- and Y-directions, to control three axes including X, Y and .theta.. This is called an orthogonal coordinate system.
Further, in a horizontal multiple-joint robot, an upper arm is moved angularly by a shoulder joint through .theta..sub.1, a front arm is moved angularly by an elbow joint through .theta..sub.2, and a hand is rotated by a wrist joint through .theta..sub.3, to control three axes including .theta..sub.1, .theta..sub.2 and .theta..sub.3, thereby obtaining a position and a posture (X, Y, .theta.). The following relationship exists between .theta..sub.1, .theta..sub.2 and .theta..sub.3 and X, Y and .theta.: EQU X=l.sub.1 cos .theta..sub.1 +l.sub.2 cos (.theta..sub.1 +.theta..sub.2) EQU Y=l.sub.1 sin .theta..sub.1 +l.sub.2 sin (.theta..sub.1 +.theta..sub.2) EQU .theta.=.theta..sub.1 +.theta..sub.2 +.theta..sub.3
In this manner, in the prior art, the arrangement is such that a first axis is mounted on a motion of a second axis, and the last axis is mounted on an assembly including the first and second axes. For this reason, the axis in the lowermost layer must move drive mechanisms and guide mechanisms for the respective upper two layers including the second and third axes. Thus, a large power is required.
Further, in the orthogonal coordinate system, not only an area approximate to an area twice strokes required are necessary, but also a considerable thickness is required because of three layers. Furthermore, there is a problem that the orthogonal coordinate system cannot obtain sufficient speed as compared with the horizontal multiple-joint robot.
On the one hand, only a small area is required for the horizontal multiple-joint robot. However, there is such a problem that rigidity and accuracy cannot be obtained as compared with the orthogonal coordinate system, because of a cantilever.